Sassafras tzumu is one of the most valuable tree species in southern China. However, the dormancy of the seed limits its seedling quality. In order to improve the germination percentage of seeds, characteristics, causes of dormancy, changes in endogenous hormones and nutrients during storage were investigated. The results showed that seed viability was 78.11%, and the highest water absorption rate was 26.09%. The germination percentage of the embryo without cotyledon was 21.13%, but intact seed and embryo could not germinate on hormone-free medium. Seed embryo and coat extracts were found to have a significant inhibitory effect on seed germination. Germination percentage were significantly positively correlated to free amino content (p < 0.01) and negatively correlated to soluble sugar content (p < 0.05) during storage. The levels of GA3 and GA3/ABA had significant effects on the germination percentage of S. tzumu seed storage at 4°C. The results suggest that S. tzumu seeds are in intermediate physiological dormancy at maturity and are mainly caused by the presence of inhibitory substances in the seed tissues. Furthermore, changes in endogenous hormones and metabolism of nutrients at 4°C can significantly promote the release from dormancy of S. tzumu seeds.